continuous body of salt water that is contained in enormous basins on Earth’s surface. When viewed from space, the predominance of Earth’s oceans is readily apparent. The oceans and their marginal seas cover nearly 71 percent of Earth’s surface, with an average depth of 3,795 metres (12,450 feet). The exposed land occupies the remaining 29 percent of the planetary surface and has a mean elevation of about 840 metres (approximately 2,755 feet). Actually, all the elevated land could be hidden under the oceans and Earth reduced to a smooth sphere that would be completely covered by a continuous layer of seawater 2,686 metres (8,812 feet) deep. This is known as the sphere depth of the oceans and serves to underscore the abundance of water on Earth’s surface. Earth is unique in the solar system because of its distance from the Sun and its period of rotation. These combine to subject Earth to a solar radiation level that maintains the planet at a mean surface temperature of 17 °C (62.6 °F),...